The present invention relates to a semiconductor device and, more particularly, to a technique which is especially effective when applied to a large scale integrated circuit device including a data output buffer and a data input buffer and to an electrostatic breakdown preventing technique by a charge device model of the large scale integrated circuit device.
The electrostatic breakdown of a device is caused by Electro-Static Discharge (ESD). This ESD phenomenon is classified into the following three: (1) Human Body Model (HBD); (2) Charge Device Model (CDM); and (3) Field Induced Model. Of these, the charge device model includes the Charge Package Model (CPM). Of these, the field induced model is not considered to be serious because it hardly occurs.
In the human body model, an electrostatically charged human body comes into contact with a device so that the charge of the body is released to a pin of the device. If, at this time, another pin is connected to a potential, for example, to the earth, the discharge current flows through the device, resulting in breaking it down. A similar phenomenon occurs if a charged human brings a pin of a device into contact with a metal plate while holding the device with his or her hand. In a test circuit of this human body model, a capacitor likened to a human body is charged, and a resistor likened to the skin resistance is connected to a pin of the device. And, a voltage pulse is applied. A protective circuit premised by the human body model is disclosed in Japanese Patent Laid-Open No. 128872/1993.
As the automation technology has developed in recent years, devices are hardly handled by the human hand, so that the necessity for taking measures against the ESD breakdown according to the human body model is becoming lower and lower. As a result of the automation of the IC testing step and the device assembling step, on the contrary, devices tends to be frequently charged by friction of its package in transporting the device and by the contact with charged manufacturing apparatuses which is likened to the charge device model.
According to this charge device model, as shown in FIG. 18, when a device (LSI) approaches charged insulator, all its conductors (including the chip, the lead frame and the wires) are uniformly charged by the electrostatic induction, as shown by A. When the induced charge is released, as shown by B, an electrostatic breakdown occurs. Alternatively, as a result of the release of the induced charge, as shown by B, the device undergoes actual charge, as shown by C. When a lead is grounded to the earth, as shown in FIG. 19, a discharge occurs, causing an electrostatic breakdown. Thus, the ESD breakdown according to the charge model is caused with the result that the uniform charge in the conductors of the device is concentrated on the pads of the discharge pins.